1. Field of the Invention
The present invention relates generally to a developing method of developing an electrostatic latent image formed on an image carrying member in an image forming apparatus such as a copying machine or a printer, and more particularly, a developing method of conveying a developer containing toners and carriers to a developing region opposite to the image carrying member by a developer conveying member and supplying the toners in the developer from the developer conveying member to the image carrying member in the developing region to carry out development.
2. Description of the Related Art
In an image forming apparatus such as a copying machine or a printer, various methods have been conventionally known as a developing method of supplying toners to a latent image formed on an image carrying member to carry out development. As a representative of such developing methods, a developing method so adapted as to expose the surface of a charged image carrying member to form an electrostatic latent image on the surface of the image carrying member, while conveying a developer containing toners and carriers to a developing region opposite to the image carrying member having the electrostatic latent image formed thereon in the state of a magnetic brush by a developer conveying member, bring the developer on the developer conveying member into contact with the surface of the image carrying member in the state of the magnetic brush in the developing region, and supply the toners in the developer from the developer conveying member to a latent image portion of the image carrying member to carry out development has been widely known.
In a case where the developer is thus brought into contact with the image carrying member in the state of the magnetic brush to carry out the development, however, there are some problems. For example, the toners supplied onto the image carrying member are scraped by the magnetic brush on the developer conveying member, so that a toner image formed on the image carrying member is distorted. Particularly in a case where toners in a plurality of colors are successively supplied to the image carrying member to carry out multicolor development, the toners in a color previously supplied to the image carrying member are scraped by the contact with the magnetic brush so that the image is distorted when the toners in the subsequent color are supplied to carry out the development, and the toners in the other color are independently mixed with the toners previously supplied Consequently, it is impossible to carry out good multicolor development in accurate colors.
The fact that the magnetic brush is hard because the magnetic force of the carriers in the developer is strong, and charges remaining on the carriers when the toners in the developer are supplied to the image carrying member, that is, so-called counter charges are considered the reason why the toner image formed on the image carrying member is distorted in a case where the developer is brought into contact with the image carrying member in the state of the magnetic brush to carry out the development.
Therefore, it has been conventionally considered that carriers having a low magnetic force are used as the carriers in the developer, and the bristles of the magnetic brush in contact with the image carrying member are softened, to prevent the toner image from being distorted by the contact of the magnetic brush.
When the carriers having a low magnetic force are thus used, however, the binding force of the carriers on the developer conveying member is weakened, so that the carriers are separated from the developer conveying member to easily adhere to the image carrying member. Particularly when an image having a high frequency such as a ladder pattern or an image such as a kanji character pattern formed with a large number of strokes is developed as an input image, the carriers adhering to the image carrying member are increased in number.
When the carriers thus adhere to the image carrying member, the carriers, together with the toner image, are transferred to paper, so that omissions due to the adhesion of the carriers occur in a formed image, and the image carrying member is damaged by the adhering carriers, causing some problems. For example, stripe-shaped noise or dot-shaped noise is produced in the formed image.
In recent years, in order to prevent the toner image formed on the image carrying member from being distorted by the magnetic brush formed of the developer as described above, a method of conveying a two-component developer containing toners and carriers to a developing region opposite to an image carrying member by a developer conveying member, exerting a oscillating electric field on the developing region, and supplying the toners in the developer from the developer conveying member to the image carrying member in a non-contact state where the developer is not brought into contact with the image carrying member, to carry out development has been developed, as disclosed in Japanese Patent Laid-Open No. 32858/1986, Japanese Patent Laid-Open No. 182760/1987, etc.
Even when the oscillating electric field is thus exerted on the developing region, and the toners in the developer are supplied to the image carrying member in a state where the developer is not brought into contact with the image carrying member, to carry out the development, however, counter charges remain on the carriers by the supply of the toners, and the carriers are attracted to the image carrying member by a wraparound electric field based on a potential difference between an exposed portion and an unexposed portion in the image carrying member, so that the carriers still adhere to the image carrying member. Further, an edge portion of an image is strongly developed by the wraparound electric field, whereby a lot of toners are supplied, so that the edge portion of the image is thickened or deepened.
In order to prevent the carriers from thus adhering to the image carrying member, a method of increasing the amount of a developer conveyed to an image carrying member by a developer conveying member to keep the consumption rate of toners in the developer low has been considered, as disclosed in Japanese Patent Laid-Open No. 323681/1993.
If the amount of the developer conveyed to the image carrying member by the developer conveying member is thus increased, however, in supplying the toners in the developer to the image carrying member upon exertion of a oscillating electric field on the developing region to carry out development as described above, the toners scattered without being supplied to the image carrying member are increased in number, causing some problems. For example, a formed image is fogged, and the inside of the apparatus such as the copying machine is contaminated by the scattered toners.
Furthermore, if the amount of the developer conveyed to the image carrying member by the developer conveying member is increased, a lot of charged toners in the developer are not used for the development, resulting in reduced development efficiency. Therefore, a lot of charged toners are returned to a developing device by the developer conveying member in a state where they are held in the carriers. Therefore, toners newly supplied and the carriers are not sufficiently mixed and agitated, so that the new toners are not sufficiently charged.